1. Technical Field
The present invention relates to a refractometer for determining the refractive index of a liquid and, optionally, variables derived therefrom such as, for example, a sugar concentration, having a sensor system which has a radiation source for generating a measuring beam, a beam detector for detecting the measuring beam, and a measuring path to be traveled by the measuring beam, on which path a measuring surface to be wetted by the liquid and which interacts with the measuring beam is situated.
2. Description of the Related Art
Such refractometers are used as digital or analog measuring instruments, for example, for determining the concentration of certain substances dissolved in a liquid and influencing the refractive index. For example, in wine-making, the determination of the sugar content in the grape juice is an important area of application of such measurements.
Two types of measuring instruments are basically known in this area; one requires a sample to be taken and introduced into the measuring instrument, for example, drop by drop, while the other design provides for the immersion of a sensor into the analyte, i.e., into the liquid, to perform the measurement.
Sampling in the case of the first above-named variant is relatively complicated and time-consuming, and the instrument must be thoroughly cleaned before each sampling. In the instrument of the second variant, the problem often arises that the refractometer has a temperature which is different from that of the analyte, so that the required temperature compensation in determining the temperature-dependent refractive index is very difficult to perform. Furthermore, it may be difficult to take into account the effect of external light in optical measurements when a probe is introduced into the substance to be analyzed.
U.S. Pat. No. 5,859,696 describes a refractometer of a type similar to the one mentioned previously for determining the sugar content in a liquid, in which an optical beam is emitted by a radiation source, is reflected inside a transparent body on a measuring surface, and returned into a beam detector. If the measuring surface is wetted on the outside with a liquid having a high refractive index, for example, a soft drink having a high sugar concentration, this results in the refractive index of the liquid on the measuring surface approaching the refractive index of the transparent body and thus in a weaker reflection of the beam on the measuring surface or, in other words, in most of the beam penetrating the measuring surface and entering the liquid, so that the intensity of the reflected beam is greatly reduced. This is detected optically and a high or low sugar concentration is obtained as a function of the measured intensity of the reflected beam. The instrument is usable in a simple manner by immersing a measuring tip into the liquid and detecting the corresponding signal on the portable instrument. The temperature of the liquid is not taken into account in the measurement.
Accordingly, it is desirable to refine a refractometer of a simple construction of the above-mentioned type in such a way that a simple and rapid handling is made possible, while effective temperature compensation is ensured.
According to the present invention, a refractometer is disclosed in which the measuring surface is situated in a depression for samples of an insertion probe which is insertable into the liquid. The design according to the present invention makes it possible to use the insertion probe, which is insertable into the liquid and containing the depression for samples, for sampling and measurement, the refraction number, i.e., the refractive index, being determined in the depression after sampling the liquid to be measured. Only a small volume of liquid remains in the depression for samples; therefore the temperature between the insertion probe and the liquid is rapidly equalized, so that the refractometer and the liquid have the same temperature at the time of the measurement. Furthermore, temperature compensation is made simple by measuring the temperature at the probe. For this purpose, a temperature sensor may be placed within the insertion probe.
To take a new sample of the same or a different liquid, it is sufficient to insert the insertion probe into a liquid again; optionally, the depression for samples may be briefly cleaned beforehand if this seems to be necessary when handling liquids to be measured. Otherwise, it is also conceivable to simply introduce the probe into the liquid again, the liquid measured in the first measurement being simply washed out of the depression for samples by the second liquid.
Sampling and, optionally also, repeated use of the refractometer according to the present invention in a measurement is considerably simplified, and effective temperature compensation is made possible. For example, the insertion probe may have a tip at its end to make insertion into relatively large fruits possible to directly measure the fruit juice inside. The surface of the insertion probe may also have a groove through which the liquid to be measured may flow into the depression for samples after brief insertion through the liquid surface or into a fruit.
An advantageous embodiment of the present invention provides for the measuring surface to be delimited by a lens body.
The lens body is situated in such a way that one side is wettable by the liquid and on the other side the lens body surface is kept free of the liquid. The radiation source and the beam detector are then placed on the side of the lens body which is kept free of the liquid, permitting the measuring beam from the radiation source to impinge on the lens body, to be at least partially reflected there on the measuring surface wetted with the liquid, and to be subsequently directed to the beam detector. The intensity of the reflected measuring beam is then a function of the ratio between the refractive index of the lens body and that of the wetting liquid. The design of the geometry of the lens body is preferably such that the measuring beam is bundled or remains bundled in the lens body, and a suitable arrangement of the radiation source and the beam detector with respect to the lens body may be selected.
According to another advantageous embodiment of the present invention, the measuring surface is delimited by a glass body.
In principle, the fact that the measuring surface is delimited by a glass body means that cleaning of the measuring surface after performing the measurement is simplified without scratching the measuring surface. In addition, effective temperature equalization between the sensor system and the liquid is ensured by the relatively good thermal conductivity of glass.
With respect to the stability of the mechanical construction, the present invention is advantageously designed in such a way that the radiation source, the beam detector, and the lens body or, as the case may be, the glass body, are held in a metallic mount, which is made of steel or aluminum in particular.
The design of the mount made of a stable material in the form of steel or aluminum results in the geometrical configuration of the radiation source, detector, and lens being sufficiently stable to the point that even impacts will not alter the measuring path. The metallic design also makes rapid and effective temperature equalization between the individual elements of the sensor system and the liquid in the depression for samples possible. Temperature equalization may be improved by contact between the liquid and the metallic mount.
It is also advantageous to integrate a temperature sensor into the area of the sensor system. In this way the temperature of the sensor system and of the liquid may be measured at the same time as the temperature-dependent refractive index after these temperatures have adjusted to one another, which occurs after a few seconds. The measured refractive index of the liquid may then be recalculated to a normalized temperature in an analyzer, taking into account for the compensation the measured temperature. Because only a single temperature is to be taken into account with the sensor system according to the present invention, the calibration of the sensor system is also greatly simplified.
To protect the sensor system in the event of brief temperature changes, the metallic mount may be surrounded by a material, in particular by a synthetic material whose thermal conductivity is less than that of the mount material.
Normally the sensor system is protected by a plastic sheath made of a polymer or an elastomer, which of course leaves the depression for samples and the measuring surface free. The liquid sample taken and the sensor system are largely temperature-equalized independently of the ambient temperature due to the thermal contact in the depression for samples, and the refractive index is measured at this temperature. This temperature is measured simultaneously inside the refractometer in the area of the sensor system, preferably by a temperature sensor, in order to be able to take temperature influences into account and to refer the measurement result to a normalized temperature.
According to another advantageous embodiment of the present invention, the lens body or the glass body is made of a material having a refractive index which is greater than 1.5, in particular greater than 2. The refractive index of the lens is preferably 1.85.
A high refractive index of the glass body is advantageous when liquids also having high dielectric constants or refraction indices are to be measured.
The depression for samples advantageously has a volume of less than a milliliter for a particularly rapid temperature equalization.
The sensor system may be advantageously designed in that the radiation source is formed by an infrared LED and the beam detector is formed by a semiconductor which is sensitive in the infrared range.
Interference by external light is normally very small in the infrared range, and the components used operate reliably and relatively unaffected by errors.
The refractometer advantageously has a lens body which has an area having greater curvature, which faces the radiation source and the beam detector, and an area having lesser curvature which delimits the measuring surface.
This design of the lens body ensures optimal guidance of the measuring beam and optimum design of the measuring surface in terms of its metrological characteristics, the measuring surface also being easy to clean.
The present invention is elucidated in the following on the basis of an exemplary embodiment illustrated in the drawing.